An LED is a light-emitting semiconductor device based on the electroluminescence of the P-N junction of a semiconductor. LEDs have the advantages including pollution free, high brightness, low power consumption, long life cycle, low operating voltage and small-scaled. Since the invention of the GaN-based LED in the 1990s, with the developments in the studies, the brightness of LEDs has been continuously improved, and the application range has become increasingly wider. It seems inevitable that GaN-based LEDs will replace the existing lighting equipment. However, for the popularization of semiconductor lighting, a number of problems are still left to be solved, among which the most important one lies in production cost and luminous efficiency.
The structures of an LED can be classified into two categories: lateral and vertical. A high-power GaN-based LED with a lateral structure in which sapphire is used as a growth substrate has the disadvantages including low heat dissipation efficiency, current crowding, low current density and high production cost. In order to solve the heat dissipation problem of the high-power GaN-based LED with a lateral structure, the technique of flip chip has been proposed. However, the processing steps of flip chip are complex and the production cost is high. A conventional GaN-based LED with a vertical structure in which a SiC wafer is used as the original growth substrate includes two electrodes at respective sides of the growth substrate, and has the advantages such as good heat dissipation efficiency, uniform current distribution, improved current crowding, increased current density and effective utilization of the material of the light-emitting layer. A conventional GaN-based LED with a vertical structure in which sapphire is used as the original growth substrate includes two electrodes at respective sides of a support substrate, and has the advantages such as high heat dissipation efficiency, uniform current distribution, improved current crowding, increased current density, effective utilization of the material of the light-emitting layer and high light-extraction efficiency. Sapphire is an electrical insulating material, and hence lift-off of the growth substrate is needed. Nichia Corporation in Japan and Osram in Germany have developed techniques for laser lift-off of the sapphire substrate used in the manufacturing of LED chips with a vertical structure. These techniques can effectively solve the heat dissipation and light escape problems, make it possible for a microstructure to be formed on an N-face, improve light extraction efficiency, and allow for reuse of the sapphire. By employing the laser lift-off techniques in conjunction with bonding technology, the epitaxial layer of a GaN-based LED can be moved onto a substrate having a high electrical conductivity (e.g., Si, Cu, or Al), thereby avoiding undesired effects by the sapphire substrate on the GaN-based LED. However, this approach has the following problems: (1) laser lift-off may crack the GaN film internally, causing electrical leakage; (2) the heat generated in the process of laser lift-off of a sapphire substrate is high, since the wafer bonding layer is just several micrometers away from the interface between the sapphire substrate and GaN, the bonding layer is likely to be affected (e.g., remelt); and (3) the laser lift-off techniques are not compatible with earlier processing processes, and the machines and tools are costly.